Question 10 [2+2+2 +2 + 2 = 10 marks] The Cosmic Microwave Background (CMB) is blackbody radiation remaining from an early hot phase in the universe, and currently has a temperature of 2.7K. First we will determine the current number density of CMB photons. (a) What is the current energy density of the CMB? (b) What is the average energy of a single photon in the CMB today (at the peak of the blackbody curve)? (c) If we take this as the typical energy of a CMB photon, how many CMB photons are there in a cubic metre today? (d) Suppose that Q2 = 0.04, and Perit = 9.2×10-30 g/cm³. How many nucleons (either m protons or neutrons) per cubic metre are there in the universe today? (e) What is the ratio of photons to nucleons in an average part of the universe today?